OF HRYOZOA. 



275 



Fig. 24. CELLS OF CHEILOSTOMATOU8 

 BKYOZOON. (Enlarged after Busk.) 



c. Animal Cell ; I, Lid or Oi>erouHiin ; o, Oviccll ; d, 

 lanaerted Avicularia. 



and 



species to which he gave the mime of the " birds'-head Coralline." They are known as " avicularia," 

 from the strong resemblance the most perfect present to a bird's beak, and are always composed of a 

 chamber lodging muscles, and sometimes a tiny tuft of bristles seated on a prominence - 

 possibly a tactile organ of sense a more or less developed mandible, and a horny beak which can be 

 brought into, and withdrawn from opposition, by means of two sets of muscles. The avicularia are of 

 three distinct types, progressing from the rudimentary form of a 

 dwarfed cell of the colony with an enlarged lid or operculum "the 

 immersed " (Fig. 24, B, d) to those seated on the cell wall with a 

 small chamber and mandible the "sessile" (Fig. 25, B) up to the 

 ultimate type," the pedunculate " (Fig. 25, c), which are situated on a 

 movable jointed stalk, look like a bird's head, and sway to and fro 

 snapping their jaws incessantly. These stalked forms have been 

 credited with an alimentary function, for they seize small or- 

 ganisms, usually worms, and retain them pertinaciously in spite 

 of their vigorous efforts to escape. But it is evident that they 

 are not able to convey them to the mouth, which is, moreover, 

 too small to swallow the objects generally captured. In accor- 

 dance with the views of Mr. Hincks, the ciliated tentacles are 

 sufficient to secure regular nourishment for the animal, and the 

 avicularia are charged with a purely defensive function. " They 

 may either arrest or scare away unwelcome visitors. Their 

 vigorous movements, and the snapping of their formidable jaws, may have a wholesome and deterrent 



effect on loafing annelids, and other vagrants ; whilst the 

 occasional capture of one of them may help still further 

 to protect the colony from dangerous intrusion." 



The avicularia, however diverse in size, shape, 

 position, may all be regarded as 

 metamorphosed " animal cells." 

 The operculum becomes first 

 modified until through succes- 

 sive stages the true " bird's head " 

 form is developed, and the cell 

 contains merely the machinery by 

 which the mandibles are worked. 

 In this state they present, as Pro- 

 fessor Huxley has shown, a re- 

 markable resemblance to the valves 

 of the Brachiopod shell, in which 

 the arrangement of the muscles, 

 and the articulation of the two 

 valves, correspond closely with the 

 movable jointed mandible of the 

 bird's beak in the operculated 

 Bryozoa. The " avicularia " are 



also linked by a series of transitional forms with the vibracular organs, 

 which likewise consist of a chamber lodging muscles, and a movable bristle. F-'g- 26. CELLS OF BUGILA 

 Mr. Hincks describes the vibracula? as acting sometimes independently of ULAKIA if\ HOLDING A 

 each other, and at others in combined action, as though swayed by a sudden WORM. (After Busk.) 

 impulse, yet with the perfect regularity and order of a machine. From this 

 fact he derives an argument in favour of some kind of colonial sensation and 

 means of communication. The occasional simultaneity of the movements of the vibraculav was 

 also noted by Mr. Charles Darwin,* who gives an interesting description of both kinds of the 



"Voyage of the Beagle," p. 201. 



Fig. 25. A, PORTION" OF POLYZOAKIUM WITH 

 VIBHACULJE, V. (After Hincks.) B, SESSILE, C, 

 PEDUNCULATE AVICULARIA. (Enlarged after Busk.) 

 a, Mandibl", 6, Beak.c, Chamber, m, Muscles, p, Peduncle. 



c, Cell, o, Ovicell, t. Tentacles, 

 d, Avicularia. 



